1. Field of the Invention
The present invention relates to a system for sensing the air gap between a cutting tool and a work piece mounted on a machine for the purpose of controlling the traverse feed rate of the machine as contiguity approaches.
2. Description of the Prior Art
The adaptive control technique for numerically controlled machine tools contemplates the maintenance of an optimum spindle power, and accordingly the feed rate, and sometimes speed, are varied up and down to compensate for such parameters as: variations in width and depth of cut, hardness of materials etc.
The programmed tape for conventional numerically controlled machine tools, in addition to positioned information, includes feed rates and speeds which are fixed over one or more blocks of intelligence. Usually the programmer is cautious and programs a conservative feed rate so that the machine is not working to full capacity. The adaptive control technique proposes to modify the feed rate (and sometimes the speed) to increase productivity at no sacrifice to the life of the components. For example adaptive control insures that the horse power limitations of the feed motors will not be exceeded and that the cutting tool will not be damaged by exceeding the breakage feed limit.
In machining operations the cutting tool is moving through air for a considerable portion of time. Instead of the relatively slow traverse feed provided by the punched tape, it is better to move through air at rapid traverse feed and then reduce the feed upon impact of cutting tool and work. In order to provide this reduction in feed rate, the prior art teaches the use of torque transducers which sense the increased torque requirements when the cutting tool comes in contact with the work piece. Various forms of torque transducers have been developed so that the machine table feed rate can be controlled to a programmed spindle torque. When the cutting tool is in air, the feed rate is increased to a maximum or say three times the feed rate. When the cutting tool makes contact with the work, the torque transducer senses the increased cutting requirements, and the adaptive control loop drops the feed rate to a magnitude equal to the programmed torque.
Optimally when the cutter is in air, the feed rate should be at rapid traverse speed in the order of 200 inches/min instead to typically 15 inches/min. A milling machine servo is capable of decelerating from rapid traverse speed to a halt in 0.2 sec. Thus if the torque transducer senses a torque which exceeds the programmed torque, a stop signal is sent to the control section to stop the machine table. In such a situation as has been supposed, the cutter would overshoot by 0.33 inch in 0.2 sec. ##EQU1## Obviously this would spoil the workpiece and possibly chip or even break the cutting tool.
The present invention proposes to cut down on pre-positioning time by moving at rapid traverse feed through air and continuously sensing the air gap between cutting tool and work piece as contiguity approaches, so that at some pre-set point the feed is reduced to an allowable impact feed rate, allowing the power adaptive control loop to take over.